1. Field of the Invention
The invention relates to a non-dispersive infrared (NDIR) gas analyzer for verifying a measuring gas component in a gas mixture, having an infrared radiation source for generating an infrared radiation, a modulation device for modulating the infrared radiation, a measuring vessel containing the gas mixture and being irradiated by the modulated infrared radiation and a detector device arranged behind the measuring vessel in the radiation direction, where the detector device supplies a measuring signal which corresponds to the concentration of the measuring gas component in the gas mixture.
2. Description of the Related Art
Single-beam and two-beam NDIR gas analyzers are known. With single-beam devices, the infrared radiation generated by the infrared emitter is routed after modulation, such as by a rotating diaphragm wheel, through the measuring vessel containing the gas mixture with the measuring gas component to the detector device. With two-beam devices, the infrared radiation is subdivided into a modulated measuring radiation passing through the measuring vessel and into an inversely-phased modulated comparison radiation passing through a comparison vessel filled with a comparison gas. Optopneumatic detectors filled with the gas components to be verified and comprising one or more receiver chambers arranged adjacent or to the rear of one another are usually used for the detector device.
To ensure functional reliability, NDIR gas analyzers must be able to diagnose certain faults, depending on the level of safety requirement to rule out faulty measurements. The quantitative analysis of the components of an NDIR gas analyzer shows that the infrared radiation source has the highest error rate. This component is subjected to a slow but consistent ageing.
DE 35 29 482 A1 discloses a two-beam NDIR gas analyzer. Here, a further flow or pressure-sensitive sensor is arranged in a cable assembly connecting the receiver chambers in the measuring and comparison radiation path to monitor the full functionality of the gas analyzer calibrated to zero and to exclude long-term drifts of the zero point which occur as a result of the emitter ageing or window dirt, where the sensor generates a further detector signal which is proportional to the overall intensity of the detected infrared radiation. With this further signal, the functionality of the gas analyzer can also be monitored in cases of zero compensation of the gas analyzer, if, in other words, the measuring signal is zero. The further signal can be used as an input variable for regulating the infrared radiation source, whereby the radiation output is regulated such that this signal always remains constant, i.e., in cases of zero compensation, the system always has a constant sensitivity, since the influence of the emitter ageing and dirt deposits in the optical radiation path is compensated by the regulation. The otherwise necessary calibration with calibration gas at certain intervals can thus be avoided.
In contrast, with single-beam devices, a regular calibration with calibration gas is necessary, as a result of which the availability of the gas analyzer for measurement purposes is reduced.